1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly to an apparatus and a method for estimating a carrier to interference and noise ratio (CINR), which is a basic factor for estimating reception performance of a communication system using an orthogonal frequency division multiplexing (OFDM) technique or an orthogonal frequency division multiple access (OFDMA) technique.
2. Description of the Related Art
As is generally known in the art, the OFDM technique is used to transmit high-speed data through wired/wireless channels. More specifically, the OFDM technique uses a plurality of carriers to transmit data while converting serial data into parallel data and modulating each of the parallel data into a plurality of sub-carriers having orthogonality, that is, sub-channels.
Recently, the OFDM technique has been widely used in various digital data transmission fields, such as digital/audio broadcastings, digital TVs, wireless local area networks (WLANs), wireless asynchronous transfer modes (WATM), and broadband wireless accesses (BWAs). Conventionally, the OFDM technique could not be used because it requires a complex hardware structure. However, as various digital signal processing technologies including fast Fourier transform (FFT) and inverse fast Fourier transform have been developed, the OFDM technique can be used in the various data transmission fields.
The OFDM technique is similar to the conventional frequency division multiplexing (FDM) technique. Among other things, the OFDM technique transmits data while maintaining orthogonality between sub-carriers, thereby obtaining an optimal transmission efficiency when transmitting high-speed data. In addition, the OFDM technique may achieve high frequency efficiency and represents a superior property against multi-path fading. Further, because the OFDM technique overlaps the frequency spectrums, the OFDM technique is resistive to frequency-selective fading and reduces interference between symbols by using guard intervals. In addition, in view of hardware, it is possible to simplify a structure of an equalizer while reducing impulse-derived noise.
In an OFDM/OFDMA (hereinafter, commonly referred to as “OFDM”) system, channel signal quality, such as CINR (carrier to interference noise ratio), which is an essential parameter for an adaptive power control or adaptive modulation/coding, must be measured. According to the adaptive power control or an adaptive modulation/coding device, power or a modulation/coding level is controlled based on the quality of channels by using a CINR value. The CINR value is obtained by dividing a sum of signal powers of sub-carriers by a sum of interference power and noise. The CINR value is a basic factor for determining the quality of channels in an OFDM system.
If the channel is estimated using a sliding average window (SAW) in the OFDM system, power is defined by a square value of an absolute value of a SAW channel estimation value. Accordingly, total signal power is represented as a sum of square values of absolute values of SAW channel estimation values. In addition, if the SAW channel estimation value is subtracted from the receiving signal of each sub-carrier, only noise and an interference signal remain. Therefore, a sum of noise and interference power is represented as a sum of square values of absolute values of noise and interference values. Accordingly, in the OFDM system for estimating channels using the SAW, the CINR can be obtained by dividing the sum of square values of absolute values of SAW channel estimation values by the sum of noise and interference power.
However, the SAW channel estimation values are inaccurate values. Therefore, if the CINR estimation value is obtained using an inaccurate SAW channel estimation value, the CINR estimation value may be represented higher than a real CINR value, which is called an “error floor phenomenon”. Therefore, it is difficult to precisely estimate the real CINR value.
In addition, such an inaccurate CINR value may deteriorate performance of the adaptive power control or the adaptive modulation/coding device.